When molecules are heated to cause them to undergo chemical reaction, the molecules are excited to many different excitation states. Some states may lead to the desired product while other states lead to undesired by-products or to no reaction at all. It has only recently been found that a laser can be used to excite molecules to very specific excitation states. This is possible because laser light is nearly monochromatic, and, unlike thermal excitation, the molecules are excited by photons of nearly uniform energy and therefore can be excited only to excitation states of the same energy. From these specific states the reaction between the molecules or with other molecules can proceed only in certain well-defined directions. For example, isotopes can be separated because the excitation levels differ slightly for each isotope. The laser excites only one isotope which reacts with another type of molecule and can be chemically separated.
Reactions which would not occur at all under thermal excitation or which would be too expensive under thermal excitation are now often possible or economical using laser chemistry.
While the promises of laser chemistry have caused great interest among scientists, there are still some difficulties which must be overcome before laser chemistry can be used successfully for many chemical reactions. One of the principal problems is the destruction of the specificity of the excitation state. Clearly, if the wavelength of the laser is too broad unwanted excitation states may result which produce unwanted products. However, there are also other ways in which the molecules can be subjected to a broader band of excitation energy than is desired.
First, excited molecules can collide with each other or the reaction product and transfer energy therebetween, resulting in molecules having more or less than the desired energy. Secondly, if the molecule is moving towards or away from the light at the time that it is excited by the light, it will "see" the light as having a greater or lesser frequency, respectively, than it actually has and will be accordingly more or less excited than is desired. These two effects, intermolecular collisions and the Doppler effect, set important limitations on the type of reactions which can be successfully accomplished in laser chemistry.
In addition to line broadening, laser chemistry must also deal with the problem of removing the end products from the reaction zone as quickly as possible to prevent them from reacting with the excited or unexcited molecules to produce undesirable products.